Composites of porous film and spunbonded polypropylene useful as construction fabric such as housewrap are known. Porous melt-processed polymeric films can be made by various processes including three basis types of processes. The three process categories are processes for stretching films of neat, unblended polymers containing no fillers other than typical stabilizer additives; processes for making films from blends of two or more polymers, or from a blend of a polymer with mineral oil or an organic salt in which the dispersed phase can be extracted with the film stretched either before or after the extraction; and processes for casting films made from a polymer that has been blended with a filler such as calcium carbonate or barium sulfate with the film stretched after casting.
In the first category, a common method of producing such films involves drawing or stretching a crystalline, elastic starting film to about 10 to 300 percent of its original length as exemplified by U.S. Pat. Nos. 3,839,240 and 4,138,459. This drawing operation is said to produce a microporous film ordinarily having elongated, slit-like pores with pore sizes up to 1.2 microns. U.S. Pat. No. 3,920,785 describes post-treating a stretched film with an organic solvent to increase the gas transmission rate of the film. U.S. Pat. No. 4,105,737 describes a process for producing a porous film by forming many fine cracks in a stretchable polymer composition having a fine phase separation structure, heating the polymer composition to its stretching temperature under tension so that the fine cracks do not disappear, and enlarging the cracks by stretching the film at the stretching temperature. U.S. Pat. No. 3,839,516 describes a process for swelling a polyolefin film by immersing the film in a solvent such as toluene or benzene, stretching the polyolefin film in the swollen state and drying the film under tension to form pores.
In the second category, U.S. Pat. No. 3,956,020 describes a process for dissolving a benzoate salt from a polymer article to form an ultrafine porous article. U.S. Pat. No. 4,076,656 describes a process to incorporate a water soluble liquid into a polymer and to extract the liquid with water. In U.S. Pat. No. 3,607,793, a hydrocarbon liquid is extracted from a polymeric gel. U.S. Pat. No. 3,407,253 describes a process to form sheets from blends of polypropylene and an elastomer such as polyisobutylene which are drawn to create internal voids, thereby imparting some breathability. U.S. Pat. No. 3,969,562 describes a process of blending two crystalline polymers together, extruding a sheet, cold stretching the sheet to open up crazes and then hot stretching the sheet biaxially to increase porosity. U.S. Pat. Nos. 4,116,892, 4,153,751 and 4,289,832 describe a process of extruding incompatible polymer blends into a sheet and then stretching the sheet by drawing it over a grooved roller at low temperatures to generate porosity. EP 0 273 582A describes a process in which polypropylene is blended with mineral oil and a nucleating agent and a cast film is produced. The mineral oil phase separates as droplets within the polypropylene matrix, and the droplets are removed by running the film through an extraction bath. EP 0 258 002A describes a process in which a hydrophilic polymer such as polyethylene oxide is crosslinked with UV radiation, with uncrosslinked polyethylene oxide extracted with water and the film is dried to produce a film containing pores filled by interconnecting plugs of polyethylene oxide.According to this publication, the moisture vapor transmission rate (MVTR) of the formed material is typically 10,000 g/m.sup.2 /24 hr or greater.
In the third category, U.S. Pat. No. 3,844,865 describes a process in which blends of a polymer and an inorganic salt, such as calcium carbonate, are formed into films and stretched uniaxially or biaxially to obtain a high MVTR. U.S. Pat. No. 3,376,238 describes a process in which blends of polyethylene with sugar, starch and silica gel are prepared and cast into film with the film crosslinked with peroxide or radiation, and then pore formers are extracted at elevated temperatures with the films optionally biaxially stretched before an extraction step.
Spunbond polymeric nonwoven webs can be produced by extruding a multiplicity of continuous thermoplastic polymer strands through a die in a downward direction onto a moving surface where the extruded strands are collected in a randomly distributed fashion. The randomly distributed strands are subsequently bonded together by thermobonding or by needlepunching to provide sufficient integrity in a resulting nonwoven web of continuous fibers. One method of producing spunbond nonwoven webs is disclosed in U.S. Pat. No. 4,340,563. Spunbond webs are characterized by a relatively high strength/weight ratio, isotropic strength, high porosity and abrasion resistance properties. Spunbond nonwoven webs are nonuniform in properties such as basis weight and coverage.
U.S. Pat. No. 4,766,029 discloses a house wrap material consisting of a three-layer, semi-permeable nonwoven laminate having two exterior layers of spunbond polypropylene and an interior two-component melt-blown layer of polyethylene and polypropylene in which the laminate is calendered after formation so that the polyethylene melts and flows to close up the interstitial spaced and bond the layers together.
U.S.Pat. No. 4,684,568 discloses a process for preparing fabrics that are permeable to moisture vapor and impermeable to liquid water consisting essentially of the steps of applying a continuous coating of polypropylene to a surface of a vapor and liquid permeable, fibrous sheet and then calendering the coated surface. The fabrics are disclosed as being suited for use as roofing-tile underlayment and building air-infiltration barriers.
European Patent Application No. 0 306 818 discloses a process for producing a barrier fabric which is permeable to liquid vapors and impermeable to liquids by laminating a vapor impermeable film to a reinforcing porous fibrous film such as a spunbonded polypropylene to form a composite web and then needing the film of the composite web with tapered needles to form a plurality of micropores through the film.
European Patent Application No. 0 288 257 discloses a flexible membrane useful as lining for roof or walls which is impermeable to liquid water but permeable to air and water vapor made of two spunbonded polypropylene layers adhesively bonded together by a discontinuous and porous layer of polyolefin material.
A major limitation of nonwoven composites comprising spunbond webs is that the spunbond web used to impart strength to the composite, especially in the cross-machine direction, is typically nonuniform in basis weight and coverage such that relatively "thick" and "thin" areas are easily recognized by the human eye. In many applications, attempts have been made to compensate for these poor fabric aesthetics and limiting physical properties that result from this nonuniformity of basis weight and coverage by using spunbond webs having a heavier basis weight than would normally be required by the particular application if the spunbond web had a more uniform coverage and basis weight. This, of course, adds to the cost of the composite product and contributes to greater stiffness, increased bulk and other undesirable features.
In view of the limitations of spunbond nonwoven webs in multi-layer nonwoven web composites, there is a need for improved nonwoven web composites and, in particular, those wherein a self-bonded, fibrous nonwoven web having very uniform basis weight and balanced physical properties is used for at least one layer and is adhered to at least one layer of a porous film. These composites find particular use for vapor-permeable and liquid-impermeable product applications for example, air-infiltration barriers, house wrap, roofing-tile underlayment, covers for automobiles, motor bikes, stationary equipment and the like, and construction fabric for medical apparel, general protection and chemical protection, and liners for sporting apparel and the like.
The above patents do not disclose the invented self-bonded nonwoven web composite comprising at least one layer of a polymeric porous film adhered to at least one layer of a uniform basis weight self-bonded, fibrous nonwoven web which provides a lighter and more uniform basis weight layer of nonwoven web having vapor-permeable and liquid-permeable properties.
An object of the present invention is to provide improved composite fabric structures.
Another object of the present invention is to provide a self-bonded nonwoven web and porous film composite comprising at least one layer of a uniform basis weight self-bonded, fibrous nonwoven web comprising a plurality of substantially randomly disposed, substantially continuous thermoplastic filaments wherein the web has a Basis Weight Uniformity Index (BWUI) of 1.0.+-.0.05 determined from average basis weights having standard deviations of less than 10% adhered to at least one layer of a polymeric porous film.
The objects of the present invention are also attained with a self-bonded nonwoven web and porous film composite having a basis weight of about 0.2 oz/yd.sup.2 or greater, comprising
at least one layer of a uniform basis weight self-bonded, fibrous nonwoven web having a basis weight of about 0.1 oz/yd.sup.2 or greater and a BWUI of 1.0.+-.0.05 determined from average basis weights having standard deviations of less than 10% comprising a plurality of substantially randomly disposed, substantially continuous thermoplastic filaments, wherein the filaments comprise a thermoplastic selected from the group consisting of polypropylene, ethylene-propylene copolymer, high density polyethylene, low density polyethylene, linear low density polyethylene, polyamides, polyesters, a blend of polypropylene and polybutene, and a blend of polypropylene and linear low density polyethylene wherein said filaments have deniers in the range of about 0.5 to about 20, such layer being adhered to
at least one layer of a polymeric porous film having a basis weight of 0.1 oz/yd.sup.2 or greater and a moisture vapor transmission rate (MVTR) of about 100 g/m.sup.2 /24 hr or greater as determined according to ASTM E-96, procedure E.
The objects of the present invention are further attained with a self-bonded nonwoven web and porous film composite having a basis weight in the range of about 1.0 to about 6.0 oz/yd.sup.2, comprising
at least two layers of a uniform basis weight self-bonded, fibrous nonwoven web having a basis weight of about 0.25 oz/yd.sup.2 or greater and a BWUI of 1.0.+-.0.05 determined from average basis weights having standard deviations of less than 10% comprising a plurality of substantially randomly disposed, substantially continuous thermoplastic filaments, the filaments comprising a thermoplastic selected from the group consisting of polypropylene, a blend of polypropylene and polybutene and a blend of polypropylene and linear low density polyethylene and having deniers of about 0.5 to about 20 with the layers adhered to
at least one layer of an oriented polymeric porous film having a basis weight of about 0.5 oz/yd.sup.2 or greater comprising a propylene-based resin film made from about 70 to 100 wt % of a polypropylene having a melt flow rate of about 1.0 to about 10 as measured by ASTM D-1238 and about 30 to 0 wt % of an ethylene-propylene copolymer having an ethylene content of about 10 to about 50 wt % and a melt flow rate of about 0.5 to about 10 as measured by ASTM D-1238 and formed by the steps of:
(a) dispersing in a propylene-based resin a nucleating agent capable of producing beta-spherulites,
(b) extruding the nucleating agent-containing propylene-based resin into a film,
(c) cooling the film below the crystallization temperature of the propylene-based resin to form at least 20 wt % of beta-spherulites in the film,
(d) extracting selectively the beta-spherulites in an amount corresponding to at least 15 wt % of the propylene-based resin from the cooled film with an extraction solvent to form a porous film, and
(e) orienting the porous film by heating the porous film at a temperature of about 115.degree. to about 135.degree. C. for a time period of about 2 to about 20 seconds and stretching the heated porous film in at least one direction at a stretch ratio of about 1.5 to about 7.5 to form the oriented polymeric porous film.
Among the advantages obtained from the self-bonded nonwoven web and porous film composites of the present invention are improved hydrostatic waterproofness, improved MVTR and improved strength per unit of basis weight. The composites of the present invention have improved barrier protection from water, solvents, hazardous chemicals and blood compared to spunbond nonwovens or spunbond/meltblown/spunbond composites with the improved barrier protection being advantageous for medical apparel, general protection and chemical protection apparel and liners for sporting apparel. These improvements are achieved due to the very uniform basis weight property of the self-bonded, fibrous nonwoven web which enables these self-bonded webs to be used to provide strength to the composites and can result in lower overall basis weights of the fabrics. Improved cross-machine direction tensile strength per unit of composite basis weight is achieved due to the excellent cross-machine direction tensile strength and uniform basis weights exhibited by the self-bonded, fibrous nonwoven webs used in making the composites of the present invention. Oriented porous films having a MVTR of 100 g/m.sup.2 /24 hours or greater contribute to the improved moisture transmission rates of the composites of self-bonded nonwoven web and porous film.